A timing circuit of a controller generates a clock signal having a switching period for use by a pulse width modulation (PWM) circuit to control a switch of a power supply. The switching period of the clock signal is based on a charging time plus a discharging time of a capacitor included in the timing circuit. A first current source charges the capacitor while the timing circuit is in a normal charging mode. A second current source charges the capacitor while the timing circuit is in an alternative charging mode that is when the on time of the switch exceeds a threshold time. The current provided by the second current source is less than the current provided by the first current source such that the switching period of the clock signal is increased in response to the timing circuit entering the alternative charging mode.
Legal claims defining the scope of protection, as filed with the USPTO.
1. A controller for use in a power supply, the controller comprising, a pulse width modulation (PWM) circuit to generate a drive signal to control a switch to regulate an output of the power supply in response to a switch current flowing through the switch and in response to a clock signal having a switching period; and a timing circuit coupled to the PWM circuit to provide the clock signal, wherein the timing circuit includes: a timing capacitor, where the switching period of the clock signal is based on a charging time that the timing capacitor charges to an upper reference voltage plus a discharging time that the timing capacitor discharges to a lower reference voltage; a first current source coupled to charge the timing capacitor while the timing circuit is in a normal charging mode that is when an on time of the switch is less than a threshold time; and a second current source coupled to charge the timing capacitor while the timing circuit is in an alternative charging mode that is when the on time of the switch exceeds the threshold time, wherein a magnitude of the current provided by the second current source is less than a magnitude of the current provided by the first current source such that the switching period of the clock signal is increased in response to the timing circuit entering the alternative charging mode.
2. The controller of claim 1 , wherein the timing circuit further comprises: a first comparator coupled to compare a voltage on the timing capacitor with the lower reference voltage; and a first monostable multivibrator coupled to generate a pulse of the clock signal in response to an output of the first comparator indicating that the timing capacitor has discharged to the lower reference voltage.
3. The controller of claim 1 , wherein the timing circuit further comprises: a second monostable multivibrator coupled to output a pulse in response to the drive signal indicating the beginning of the on time of the switch, wherein a length of the pulse is equal to the threshold time; and a logic gate coupled to receive the drive signal and the pulse of the second monostable multivibrator, wherein an output of the logic gate is coupled to enable the second current source and to disable the first current source if the on time of the switch exceeds the length of the pulse.
4. The controller of claim 3 , wherein the logic gate is configured to keep the second current source enabled and the first current source disabled until an end of the on time of the switch.
5. The controller of claim 1 , wherein the switching period of the clock signal is a fixed switching period when the on time of the switch is less than the threshold time.
6. The controller of claim 5 , wherein the threshold time is equal to one-half the fixed switching period.
7. The controller of claim 1 , wherein the timing circuit increases the switching period by an amount of time that is responsive to a difference in time between the on time of the switch and the threshold time.
8. The controller of claim 7 , wherein the amount of time that the timing circuit increases the switching period is proportional to the difference in time between the on time of the switch and the threshold time.
9. The controller of claim 1 , wherein the PWM circuit is coupled to turn off the switch in response to the switch current reaching a current limit.
10. The controller of claim 9 , wherein the PWM circuit is coupled to receive a feedback signal representative of the output of the power supply and wherein the current limit is a variable current limit responsive to the feedback signal.
11. The controller of claim 1 , wherein the second current source charges the timing capacitor for a remainder of the on time in response to the timing circuit entering the alternative charging mode.
12. The controller of claim 1 , wherein the magnitude of the current provided by the second current source is less than or equal to one-half the magnitude of the current provided by the first current source.
Cooperative Patent Classification codes for this invention. Click any code to explore related patents in that topic.
August 20, 2013
March 4, 2014
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